1. Field of the Invention
The present invention relates to a dynamic speaker in which a single diaphragm is driven by double or multiple annular voice coils.
2. Description of the Prior Art
A loud speaker for converting an electrical signal of audio frequency band into an audio signal is required to exhibit a flat output sound pressure characteristic over a wide frequency band. Conventional loud speakers comprise a diaphragm mainly made of paper or a metal such as aluminum formed into a conical shape. Such a diaphragm has a conically concaved sound reflection surface, which causes ups and downs of the output sound pressure characteristic, thus making it impossible to produce a flat output sound pressure characteristic. In order to attain a construction of a flat sound reflection surface, a loud speaker made of a circular flat diaphragm has been suggested.
Such a loud speaker uses a diaphragm made of a foamed synthetic resin disc of a honey-comb metal disc covered with an aluminum foil on both sides thereof. This speaker has a flat front side and free of extraneous ups and downs, so that a flat output sound pressure characteristic is attained. Also, when this loud speaker is mounted on a baffle board, it is possible to arrange the front of the diaphragm in the same plane as the front of the baffle board, thus making it possible to make most of the effect of the baffle board.
When this flat diaphragm is reciprocated for free vibrations, however, the diaphragm is subjected to a split vibration at high frequency range, thus undesirably adversely affecting the output sound pressure characteristic. Specifically, when the diaphragm is vibrated at resonant frequency, the diaphragm is subjected to a split vibration in resonance with the particular frequency, so that some part of the diaphragm vibrates vertically while the other part thereof remains unvibrated. This split vibration occurs also at the secondary, tertiary, fourth-power, . . . N-th power frequency of the resonant frequency, with the result that many ups and downs occur in the high frequency range of the output sound pressure characteristic on the one hand and the primary split vibration occurs in a frequency range lower than the conical diaphragm on the other hand.
This split vibration of the flat diaphragm may be prevented by driving the nonvibrating part of the diaphragm, i.e., the nodes thereof, by means of a voice coil. When a circular diaphragm is subjected to a split vibration, the node where the amplitude of the diaphragm is zero takes a circular form. If this nodal circle is driven by a voice coil having the same diameter as the nodal circle, therefore, the split vibration can be prevented. By driving the tertiary mode of the nodal circle, for instance, the primary and secondary as well as the tertiary split virations may be prevented. In this case, however, since three nodal circles are driven, three voice coils are required. In view of the abovementioned fact that the split vibration of specific modes may be prevented by driving a nodal circle, it is desirable to drive a nodal circle of as high mode of power as possible.
In this way, a loud speaker in which a multiplicity of nodal circles are driven requires as many voice coils as the nodal circles driven, thus complicating the magnetic circuit for driving the voice coils. A magnetic circuit for driving such a loud speaker is disclosed, for example, in the Japanese Utility Model Laid-Open No. 52235/76 and Japanese Utility Model Laid-Open No. 52243/76 Specifications. This speaker comprises a plurality of magnets arranged perpendicular to the direction of vibration of the voice coils and also perpendicular to the length of the voice coil conductors. Another example of a loud speaker of this type, is disclosed by Japanese Utility Model Laid-Open No. 163120/78 and Japanese Utility Model Laid-Open No. 163121/78 Specifications, in which by use of a single magnet, magnetic fluxes produced thereform are passed sequentially through a plurality of gaps.
In the speaker of the first example, however, the magnetic poles may be supplied with magnetic energy only from a magnet of a size containable in a space corresponding to the gap between adjacent voice coils, so that the magnitude of the magnetic energy is limited by the space. Also the magnets are required to be magnetized in the direction perpendicular both to the direction of vibration of the voice coils and to the length of the voice coil conductor at the same time more inconveniently than when the magnets are magnetized along the direction of vibration of the voice coils.
In the case the second example in which a single magnet is used, on the other hand, the area and length of the magnet can be selected freely but, the total number of magnetic fluxes passing the gaps is made uniform, if the leakage magnetic fluxes are ignored. Therefore, it is impossible to distribute magnetic fluxes to each gap in a manner suitable for reducing the split vibration or eliminating a specific mode of vibrations.